The present invention relates to an image forming apparatus, such as an electrostatic copying machine and a printer, comprising a developing device of a rotary developing type which includes a plurality of development units attached to a rotary for conducting a multicolor development such as a full-color development. More particularly, the present invention relates to an image forming apparatus in which the replacement of a development unit of a cartridge style is conducted at a preset replacement position. Further, the present invention relates to an image forming apparatus comprising a locking means for locking a rotary in such a position that the respective developer carriers of a plurality of development units are selectively set to a development position relative to a latent image carrier.
Among conventional image forming apparatuses such as electrostatic copying machines and printers, various image forming apparatuses of a type comprising a developing device which conducts a multicolor development such as a full-color development by a plurality of development units attached to a rotary have been created. In such an image forming apparatus, the rotary is rotated to sequentially bring the respective development rollers of the development units to a development position so as to sequentially develop latent images for the respective colors on a photoreceptor, thereby achieving the image formation.
Among image forming apparatuses of this type, an image forming apparatus has been proposed which is provided near the outer periphery of the rotary with a development unit driving gear to which the driving force of a development unit driving motor is transmitted for rotating a developer carrier such as a development roller of one of the development units, brought in the development position, while each development unit has an input gear to be selectively meshed with the development unit driving gear. By meshing the input gear with the development unit driving gear at the development position, the developer carrier is driven by driving force of the development unit driving motor so that a latent image is developed with toner carried by the developer carrier.
It is technically difficult to replenish toner to the rotating development unit. There is an idea of achieving the replenishment of toner by employing a single component developer and replacing a development unit of a compact cartridge type at a predetermined replacement position. In this case, it is required to provide well workability enabling the easy replacement of the development unit. Therefore, it is preferable that the replacement position is set at a non-development position where there is no possibility of damaging the latent image carrier, not in the vicinity of the latent image carrier and that the direction of detaching the development unit is set to the axial direction or the radial direction.
By the way, during the rotary with the plural development units mounted thereon is rotated, the input gears of the development units collide with the driving gear which is rotatably disposed near the outer periphery of the rotary, thus generating impact on the both gears due to the collision. In the normal image forming operation, the rotation of the rotary stops when the development unit reaches the development position. Accordingly, the input gear collides with the driving gear at extremely low speed to come in mesh with the driving gear. Though the impact during the collision is therefore relatively small, it is desired to further reduce the impact to achieve the smooth meshing between the both gears.
To reduce the impact due to collision between the both gears during rotation of the rotary in the normal image forming operation, Japanese Patent Publication No. H4-11030 discloses that a driving gear is supported to a pivotal shaft fixed at a predetermined position near the orbit of an input gear toward the development position such a manner as to allow a rocking movement of the driving gear by an elastic member, thereby reducing the impact due to collision between the input gear of the rotary side and the driving gear during the normal image forming operation and thus achieving the smooth meshing between the both gears.
Japanese Patent No. 3129875 discloses the rotation of a rotary for detecting the absence or presence of the development unit, that is, the rotation of the rotary in the non-image forming operation where the normal image forming operation is not conducted.
However, Japanese Patent Publication No. H4-11030 has neither disclosed nor suggested any rotation of the rotary in the non-image forming operation, for example, the rotation of the rotary for replacing a development unit as mentioned above. Therefore, the invention disclosed in Japanese Patent Publication No. H4-11030 is useful for reducing the impact due to collision between the input gear and the driving gear in the normal image forming operation, but not always useful for reducing the impact due to collision between the input gear and the driving gear during the rotation of the rotary in the non-image forming operation.
That is, in case of replaceable development units, the rotary is required to rotate to bring a development unit to be replaced to the replacement position in non-development position as mentioned above. Accordingly, the input gear of the development unit or the input gear of other development unit must pass the position where it collides with the driving gear. According to the rotation of the rotary during the operation for replacing development unit in the non-image forming operation, the input gear and the driving gear collide with each other so as to produce impact. The impact of this case affects the driving of the rotary even though the driving gear, disposed to allow its rocking movement because of the elastic member, evades away from the input gear.
In particular, in case of replacing a plurality of development units, the rotary is subjected to imbalance due to offset load generated in a state that one or more development units are removed. In this state, the impact is further increased to affect the driving of the rotary, thus not only further affecting the driving of the rotary but also increasing the load of the rotary driving motor. As the load of the rotary driving motor is increased, in case of employing a stepping motor as the rotary driving motor, the stepping motor may be stepped out, making the reliability poor.
It is desired to reduce the time required for replacing development unit as shorter as possible to achieve rapid replacement operation. However, as the rotary is rotated at a relatively high speed, the aforementioned impact is further increased. This is also a problem.
The above problems may be caused not only during the rotation of the rotary for replacing development unit but also during, for example, the rotation of the rotary for resetting and initializing the phase of the rotary after power-on.
Japanese Patent No. 3129875 has only disclosed the rotation of the rotary in the non-image forming operation, but has neither disclosed nor suggested the impact due to the collision between the input gear and the driving gear during this rotation of the rotary, that is, not considered such problems due to the impact as mentioned above.
On the other hand, as a method of sequentially positioning the rotary at a predetermined position and locking the rotation of the rotary at the position, the simplest method is utilizing the holding force of a driving motor for rotating the rotary. However, in view of the power consumption for holding operation and the holding capacity of the driving motor, it is preferable to position the rotary by some mechanical means without resort of holding force of the driving motor. In case that a single driving motor is used for driving both the development units and the rotary, it is required to provide two power transmission control means in the corresponding driving gear trains, respectively. When the driving motor drives either of the development units or the rotary, the power transmission control means in the corresponding driving gear train is connected and the power transmission control means in the other driving gear train is isolated. That is, since the power transmission control means in the rotary driving gear train is isolated when the development unit is driven after positioning of the rotary, it is impossible to utilize the holding force of the driving motor. Therefore, another mechanical means is required for positioning the rotary. Conventionally, it has been proposed that the positioning of the rotary is mechanically achieved by engaging a lever, attached to the body of the image forming apparatus, with a part of the rotary.
The lever is disposed such that it can move between an evacuation position where it does not position the rotary and a holding position where it is engaged with the rotary to position the rotary. In this case, in case of forming a full-color image from four colors, the operation of changing color among the four colors is required for sequentially developing electrostatic latent images of four colors on a latent image carrier. Since it is preferable to shorten the time required for this operation of changing color as shorter as possible, the lever is required to rapidly move between the evacuation position and the holding position.
Accordingly, in a general conventional manner, a solenoid and a spring are used for operating the lever and are operated under consideration of the rotational time of the rotary and the energizing time of the solenoid. That is, for setting the lever to the evacuation position, the solenoid is energized to produce solenoid force, thereby rotating the lever to the evacuation position. For setting the lever to the holding position, the energization of the solenoid is cancelled and the lever is rotated to the holding position by the biasing force of a spring.
The lever shifting means is not limited to the aforementioned means employing a solenoid. The lever may be rotated by utilizing a stepping clutch or a cam driven by another moving means.
In addition, proposed in Japanese Patent Publication No. H7-117784 is an image forming apparatus of a rotary developing type employing another mechanical means for locking the rotary which is composed of a cam, attached to the rotary, and a stopping means, attached to the body of the image forming apparatus. The stopping means is always in contact with the cam to act corresponding to the rotation of the cam, thereby conducting the positioning of the rotary. In the rotary positioning means disclosed in Japanese Patent Publication No. H7-117784, the cam is formed to have a profile having a relatively sharp inclined face when the stopping means positions the rotary to the development position and a relatively gentle inclined face when the rotary rotates away from the development position.
As for the positioning means using the solenoid to operate the lever, the time from energization of the solenoid to the actual movement of a plunger of the solenoid is constant as a characteristic of a solenoid to be used. The selection of the solenoid can be made based on its characteristics according to the type of usage of the image forming apparatus.
However, since the time from the cancellation of the energization to the actual release by the plunger is not constant, it is required to set enough time for moving the lever to the holding position for the rotary from the stop of the rotary at the predetermined position. Even a case employing a shifting means for the lever other than the means using the solenoid, such as the means for shifting the lever by using a stepping clutch or a cam as mentioned above, it is relatively easy to estimate the operation time, but it is necessary to set the operation time after the stop of the rotor because the lever should be shifted to the holding position after the stop of the rotary. This operation time for shifting the lever to the holding position directly affects the operation time of changing color to make the operation time of changing color relatively longer. As a result, there is a problem that it is difficult to improve the speed of multi-color development.
In the positioning means disclosed in Japanese Patent Publication No. H7-117784, since the stopping means is always in contact with the cam, it is required to increase the contact force of the stopping means relative to the cam as the speed of the rotary for the operation of changing color is increased. This is because if the rotation of the rotary is speeded up with week contact force of the stopping means relative to the cam, the stopping means can not follow the sharp change in profile of the cam so that the stopping means may bounce and may come off the cam in the development position.
However, as the contact force of the stopping means relative to the cam is increased, the frictional force between the stopping means and the cam is increased. This frictional force may disturb the smooth rotation of the rotary. That is, the positioning means has a problem that there is a limitation on speed-up of the rotation of the rotary.
The initializing operation for detecting the position of the rotary is conducted just after power-on of the image forming apparatus. Since the rotary passes the development position without stopping at the development position during its rotation for the initializing operation, a strong rotational moment acts on the rotary driving side due to the collision of the stopping means against the cam during the stopping means passes the development position. This strong rotational moment affects the rotation of the rotary. This is a problem due to the structure that the stopping means is always in contact with the cam. Further, the direction of the reaction during the development reaction is inevitably determined by the configuration of the cam. This is a problem that the degree of design freedom of the image forming apparatus must be smaller.